A thermosensitive recording material is, in general, a material in which a thermosensitive color forming layer containing an electron donating colorless or light color dye precursor and an electron accepting color developing substance as the main components is formed on a support. When the thermosensitive recording material is heated by a heated head, a heated pen or laser beams, the dye precursor and the color developing substance react instantaneously, and a recorded material can be obtained. Development of the thermosensitive recording material has been conducted for a long time. For example, as the thermosensitive sheet for copying obtained by a specific coating treatment on the surface and comprising a composition which is colorless in the conventional form and develops color by heating or irradiation with infrared light, a thermosensitive sheet for copying in which the component developing color by reaction comprises a dye base of the lactone type, the lactam type or the sultone type having no color, an organic acid and a substance melting by heating, is proposed (Patent Reference 1). As the thermosensitive recording material which exhibits improved moisture resistance and stability in printing and can prevent coloring during drying of record-forming components in a formed coating layer and preparation of the record due to the improved moisture resistance, a thermosensitive recording material in which the record-forming unit comprises a support of a sheet material comprising a crystal violet lactone and a phenolic substance, the phenolic substance is a substance which is solid at the room temperature and, at the temperature of the thermograph, is liquefied or vaporized and reacts with the lactone to form a record, and the lactone and the phenolic substance are dispersed in polyvinyl alcohol, is proposed (Patent Reference 2).
Since the thermosensitive recording materials exhibit advantages in that records can be obtained using a relatively simple apparatus, maintenance is easy, and little noise is generated, the thermosensitive recording materials are used for thermal printers for various types of portable terminals, printers for medical images attached to ultrasonic echo apparatuses, thermopen recorders in cardiographs and analytical instruments, tickets for airplanes and trains and POS labels for merchandises.
Various properties such as excellent property for color formation, color formation with a great density using a small amount of heat, excellent properties for storage of obtained images and excellent property for maintaining whiteness in portions of no color formation are required for the thermosensitive recording materials. In particular, reliability on recorded images is important for labels for foods processed by microwave ovens, parking tickets, labels for delivery and various tickets, and excellent properties for storage such as oil resistance, moisture resistance and heat resistance are required. To satisfy the requirement, various compounds have been examined as the color developer for the thermosensitive recording material.
For example, thermosensitive recording materials comprising α,α′-bis[4-(p-hydroxyphenylsulfonyl)phenoxy]-p-xylene, α,α′-bis[4-(p-hydroxyphenylsulfonyl)phenoxy]-m-xylene or α,α′-bis[4-(p-hydroxyphenylsulfonyl)phenoxy]-o-xylene as the color developer providing a thermo-sensitive recording material which exhibits a great sensitivity, suppresses undesirable coloring of the background and exhibits excellent properties for storage which are, in particular, water resistance and resistance to plasticizers, are proposed (Patent Reference 3). As the color developer exhibiting excellent properties for storage of developed images which are, in particular, excellent resistance to plasticizers, oil resistance, light resistance and moisture resistance under heating, diphenylsulfone bridged compounds which are reaction products of dihydroxydiphenylsulfone and an alkylene dichloride or α,α′-dichloro-xylene are shown as examples (Patent Reference 4). However, the thermosensitive recording materials described above exhibit insufficient heat resistance in portions of no color formation, and the requirement for excellent storage of the color developer is not sufficiently satisfied, either.
[Patent Reference 1] Japanese Patent Application Publication No. Showa 43 (1968)-4160
[Patent Reference 2] Japanese Patent Application Publication No. Showa 45 (1970)-14039
[Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 7 (1995)-149713
[Patent Reference 4] Japanese Patent Application Laid-Open No. Heisei 10 (1998)-29969